Isomerization of Azo Compounds. Cleavage Recombination Mechanism of Azosulfides

(Z)-4-Nitrophenyl tert-butylsulfide [(Z)-4-NO2−1], upon formation of its radical anion, isomerizes to the (E)-isomer at a very fast rate of 6.3 × 106 s-1, about 103 times faster than azobenzene, while the 4-fluoro tert-butylsulfide (4-F−1) cleaves without isomerization. The particular behavior of 4-NO2−1 is related to the occurrence of a cleavage recombination mechanism, different from the usual inversion rotation mechanism. The existence of a reversible cleavage for the radical anion of 4-NO2−1 bears consequences on the mechanism of SRN1 reactions of azosulfides; the attack of the nucleophile would take place on the intermediate diazenyl radical and not on the ensuing aryl radical as previously proposed.